1. Field of Invention
The invention relates to a method of growing graphene and, in particular, to a method of growing graphene on the surfaces of the dielectrics by means of chemical vapor deposition.
2. Related Art
Graphene is a two-dimensional structure consisting of a single atomic layer of sp2 carbon atoms arranged in a honeycomb lattice. Graphene has a bond length of 0.142 nm and a thickness of 0.34 nm, and is the most basic structure among various carbon isotopes, including fullerenes, carbon nanotubes and graphite. The exotic properties of graphene, especially its high carrier mobility (5000 to 10000 cm2/Vs), hardness (1050 Gpa), thermal conductivity (5000 W/mk), current carrying capacity (108 A/cm2) and extremely large surface-to-volume ratio (2630 m2/g), have prompted research interest into graphene's applications in the next generation of bio-medical, electronic and optoelectronics devices. As a result, graphene has been considered a leading candidate for integration into conventional electronic and optoelectronic devices, and increasing amounts of capital and resources are being devoted to develop industrial fabrication and implementation methods.
Conventional approaches to obtain graphene mainly include mechanical exfoliation, high-temperature pyrolysis of silicon carbide and chemical vapor deposition (CVD). Although mechanical exfoliation can be used to fabricate high-quality single crystalline graphene on desired substrates, the size and thickness of the resulting graphene flakes are limited and cannot be scaled up. The high-temperature pyrolysis of silicon carbide, however, requires sophisticated high vacuum and high temperature (1300 to 1600° C.) conditions in the growth process, which is not cost-effective. Both approaches are costly and have trouble producing large-area graphene for the industrial applications.
Recently, CVD graphene has been successfully grown on various transition metals, such as cobalt, nickel, copper, etc. In the growth process, the carbon-containing gases are heated to 800-1400° C. and catalyzed by the transition metal to be decomposed. Depending on the various catalytic effects of the transition metals on the hydrocarbon molecules and their corresponding carbon solubility, the decomposed carbon atoms cause different degrees of deposition, dissolution and precipitation on the surface of the transition metal substrate. Nickel has a better catalytic capability, higher carbon solubility and low processing temperature (about 800 to 900° C.), but it is difficult to control the carbon precipitation rate as the temperature falls, resulting in a non-uniform thickness of the graphene film grown on the nickel surface. Copper is a less effective catalyst, with only the copper atoms on the outer layer exhibiting catalytic capability and the capacity to dissolve carbons. Thus, after the carbon source is exposed to the copper surface for decomposition, the dissolved carbon atoms immediately precipitate onto the substrate surface, followed by graphitization. When the carbon atoms covering the copper surface form a continuous graphene film, this film provides a protective shield, thus the metal surface loses its capability to catalyze the decomposition of the subsequently introduced hydrocarbon molecules. This self-limiting growth mechanism restricts the graphene layers grown on the copper surface to a 90% monolayer coverage, and represents a major breakthrough in the growth of large-area graphene films. However, a catalytic metal substrate is always necessary to obtain the high crystalline quality of graphene film obtained by chemical vapor deposition. To electrically isolate the graphene film, the metal substrate has to be removed by wet (acid) etching, and the graphene film is then transferred to an insulating substrate with the aid of a thin polymer scaffold. Unfortunately, the graphene film is often damaged by the strong acid in the etching process. The acidic chemical groups may be readily bonded to the graphene, changing its physical properties. Wrinkles or cracks may also result from the transfer process. Following the etching process residual metal particles, which scatter electrons and hence reduces electron mobility, inevitably remain on the graphene surface. In addition, prior to the etching process, an organic polymer layer has to be coated on the graphene surface to serve as a scaffold. This polymer film is usually comprised of long-chain hydrocarbons and, once these molecules come into contact with the graphene, they are difficult to be removed using any known organic solvent. The graphene surface is obscured by a thin layer of residual polymer, shielding the graphene from contact with the surrounding environment, thus throttling the high sensitivity of the graphene to environmental variation. This hinders graphene's use in applications such as sensors.
If the hydrocarbon gas is decomposed by microwaves, plasma or high-energy cyclotron electrons prior to chemical vapor deposition, graphene can be grown at a lower temperature and can be even grown on the surface of oxides or insulating layers. Thus, it is possible to bypass the wet transfer, which is an essential step in chemical vapor deposition for forming the graphene on the metal surfaces. However, due to the presence of high-energy hydrocarbon radicals, the graphene grown on the surface of the insulating substrate exhibits very high density of defects. Although the etching process is bypassed, the quality of the resulting graphene films cannot equal that fabricated by conventional chemical vapor deposition on metal substrates, thereby restricting its application and development.
Therefore, a novel technique that allows direct growth of low-defect graphene films on the surface of the insulating substrate by chemical vapor deposition would represent a significant breakthrough. Such an approach can result in large-area and high-quality graphene films as produced by mechanical exfoliation, thus allowing for practical applications in bio-medical, electronic and optoelectronic devices.